Dielectrophoresis (DEP) is a technique that employs time-varying, or alternating current (AC) electric fields to apply a force to polarizable objects. The force relies on the difference in the polarizability of the system compared to its surrounding media (e.g. water) to manipulate not only charged objects (such as DNA) but also electrically neutral objects. In solution, particles are continually subject to the bombardment of surrounding molecules and undergo Brownian motion. This thermal motion exerts an effective random force on the particle, whose maximum is roughly proportional to the inverse of its radius. For DEP to be of use, the dielectrophoretic force must overcome the randomizing thermal motion acted upon the particle. Hence, electrodes as small as possible should be fabricated to trap nano-scale objects such as nanoparticles and even single molecules. To date, however, DEP research has only been performed with lithographically created electrodes having feature sizes that limit the attainable electric field gradients.
A related major research area is the electrical measurement of single molecules. This is because single molecule electronic devices are desired for use as the basis of next generation electronic information systems. There are two difficulties which have to be overcome in order to enable charge transport on single molecules. The first is the difficulty in preparing conducting electrodes separated by a distance which matches the length of the relevant molecule. This is because molecules of interest are usually beyond the resolution limit of electron beam lithography (˜20 nm). The second is the difficulty in electrically contacting single molecules with sufficient conductivity.
A series of top-down approaches have been reported. (See, e.g., C. Dekker et al., “Nanofabrication of electrodes with sub-5 nm spacing for transport experiments on single molecules and metal clusters,” J. Vac. Sci. Technol. B 15(4), 793 (1997) and M. A. Reed et al., “Large On-Off Ratios and Negative Differential Resistance in a Molecular Electronics Device,” Science, Vol. 286, 1550 (1999)). C. Dekker made a pair of electrodes by electron beam deposition of amorphous carbon in a scanning electron microscope (SEM) chamber. Although a gap of approximately 4 nm was reported, the width of the gap should be much larger than 4 nm. Similar drawbacks are associated with the work of M. Reed et al. (M. A. Reed et al., “Conduction of a Molecular Junction” Science Vol 278, 252 (1997)). Although a gap distance within several nanometers was achieved, the width of the electrode pair was relatively large. Trapping and electrically contacting single molecule requires not only a small gap length but also a small gap width.
In addition to nanoelectrodes, nanowires are fundamental building blocks in nanotechnology. A key challenge in the application of these devices in massively parallel integrated circuits lies in their manufacturability. In particular, preceding technologies have been unable to control the spatial location of nanowires on a chip with nanometer resolution. Most manufacturing techniques consist of one of two approaches. In the first approach, catalyst sites are lithographically patterned and nanotubes, which are used as nanowires, are grown from these sites. While a significant first step, this method does not achieve resolution on the placement of nanotubes beyond the limits of lithography. Additionally, the direction in which the nanotubes grow is difficult to control. Another method consists of growing nanowires in bulk, dispersing them in solution and allowing the solution to evaporate on a solid surface. The nanowires are later contacted by lithography. While appropriate for research applications, this technique results in nanowires at random locations on a chip.
Some new techniques have been proposed and demonstrated to overcome some of these limitations. One technique uses direct current (DC) and AC electric fields to align the nanowires during or after growth. This controls their orientation and, to a lesser extent, their spatial position. A second, more promising technique is to chemically functionalize the ends of the nanowires so they bind to metal electrodes already in place. This controls their orientation, and position, although where the nanowires bind are limited by the spatial limit of how well the chemical groups can be attached to the electrodes. Additionally, there may be significant boundary resistance if the functional groups (e.g. COOH, NH2 moieties) are not conducting.